Bright and pure source of high-fidelity entangled photons for quantum computation and teleportation...

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Bright and pure source of high-fidelity entangled photons for quantum
computation and teleportation

July 15, 2004
Like virtuosos tuning their violins, researchers at the University of
Illinois at Urbana-Champaign have tuned their instruments and harmonized the
production of entangled photons, pushing rates to more than 1 million pairs
per second. The brighter and purer entangled states could assist researchers
in applications involving quantum information processing - such as quantum
computation, teleportation and cryptography - and help scientists better
understand the mysterious transition from quantum mechanics to classical
physics.

"Entangled states are the quintessential feature of quantum mechanics," said
Paul Kwiat, a John Bardeen Professor of Electrical and Computer Engineering
and Physics at Illinois. "All the manifestations of quantum mechanics in the
world around us arise from the basic but bizarre coupling that exists
between entangled particles."

For example, the properties of entangled photons are inextricably linked to
each other, even if the photons are located on opposite sides of the galaxy.
To study this "correlation at a distance," Kwiat and graduate students
Joseph Altepeter and Evan Jeffrey produce pairs of polarization-entangled
photons by passing a laser pulse through two adjacent nonlinear crystals.

"You can think of polarization as the 'wiggle' direction of the photon -
either horizontal, vertical or diagonal," Kwiat said. "As soon as you
determine the wiggle direction of one photon in an entangled pair, you
immediately know the wiggle direction of the other photon, no matter how far
apart they are."

A major production problem, however, is that entangled photons are emitted
in many directions and with a wide range of polarization phase
relationships, each acting like an individual singer in a large choir.

"Instead of hearing a soloist hit one note, we were hearing many choir
members, some of whom were singing off-key," Kwiat said.

The trick was to come up with a way of tuning the system. "We found that we
could pass the photons through another crystal - one that has a different
phase profile - to compensate for the different phase relationships," Kwiat
said. "The dissonance is corrected and the system becomes harmonized."

In the same manner as a corrector lens in a telescope removes chromatic
aberration and improves image quality, the researchers' special birefringent
crystal removes distortions in the quality of the entanglement. "After the
compensator crystal, the photons are all entangled in exactly the same way,"
Altepeter said. "We can open the iris and get more than 1 million useful
pairs per second."

Ultrabright, ultrapure sources of entangled photons are essential for
pursuing quantum computing and quantum networks, as a resource for
teleportation in quantum communication, and for sending more information
faster by means of quantum cryptography. High fidelity quantum states can
also provide researchers with a clearer picture of how the universe works on
a very fundamental level.

"Using a low-brightness source is like looking into the quantum world
through a foggy window," Altepeter said. "With a bright, pure source, we
have a very clear window that allows us to see phenomena we couldn't see
before."

The ultimate goal is to understand and develop an intuition for the quantum
nature of reality, said Kwiat, who will report the team's findings at the
International Conference on Quantum Communication, Measurement and
Computing, to be held July 25-29 in Glasgow, United Kingdom. "Higher
production rates of nearly perfectly entangled photons will help us better
understand the rules of the quantum universe, how to navigate that universe,
and how to characterize it in a very precise way."

--
Bubba Do Way Ditty

"Arbolist.... Look it up the word. I don't know, maybe I made it up.
Anyway, it's an arbo-tree-ist, somebody who knows about trees."